This invention relates to an over-current and thermal protection device, which safely prevents current flow through the device upon an excessive increase in the temperature. The.invention also relates to the use of such device for a battery cell.
Electrical devices and apparatus are placed in services in complex operating conditions where they are subject to various factors such as voltage fluctuations, current flow and temperature. These factors affect operating efficiency as well as safety. Potential concerns due to over-current, short-circuiting and overheat developed within the electrical devices and apparatus are key design issues that must be addressed.
For a battery cell, for example, a short-circuit or overcharge (for rechargeable battery cells) can cause heat generation and excessive gaseous pressure buildup within the battery. To minimize the risk of such heat generation and excessive pressure, a protection device is therefore necessary. Various means have been incorporated for use, which are designed to break the internal electrical circuit of the battery and to disrupt the current flow passing through it.
A widely used device is a positive temperature coefficient (PTC) disc installed inside the battery cell. PTC discs are thermally expansive polymeric materials filled with conductive particles, such as carbon. When the PTC heats up due to abnormal conditions, it expands in thickness and increases its resistance, reducing the current to a safe level.
Another type of protection device uses bimetal springs, as actuators, which open the circuit with the protection device in series with the apparatus protected. The actuators operate at a specified temperature due to the thermal expansion property in the bimetals to open or close the electrical pathway.
A further example incorporates a contact switch that is made out of a shape memory alloy (SMA) and coated with an electrically conductive precious metal. The contact switch opens or closes the electrical contacts by way of the changes in the metallurgical states of the SMA material. These devices tend to be expensive or difficult to manufacture due to either the material used, the processing method of the material, or the number of components in the assembly.
In view of the various designs and performance of existing commercially available devices, it would be advantageous to have a simple, inexpensive and highly functional over-current and thermal protection device.
According to the present invention, an over-current and thermal protection device is provided. The protection device has a pair of electrical conductors that are electrically insulated from each other. A contact switch is positioned between the conductors and is engageable in contact with the conductors to define an electrical conductivity pathway. The contact switch is provided with a temperature responsive actuator made of an SMA. The SMA undergoes a change in the metallurgical state when subject to a variation in temperature at a predetermined level. The variation in the temperature of the SMA could be due to excessive current (over-current) passing through the electrical conductivity pathway, or from other causes in the operating environment of the protection device. The metallurgical change in turn causes the SMA to stiffen or soften and to effect changes in the engagement of the contact switch and the electrical conductivity pathway.
In one embodiment of the invention, the contact switch is formed of an elongated member of an electrically conductive disc or conductor disc in contact with one of the conductors. The elongated member resiliently engages in contact with the other conductor to establish the electrical conductivity between the two electrical conductors. An SMA strip is secured along the elongated member and adaptable to undergo a metallurgical state change from martensite to austenite at a predetermined elevated temperature. Upon the metallurgical state change, the SMA strip stiffens and counteracts the resiliency of the elongated member to effect disengagement of the elongated member from the conductor thereby disrupting the electrical conductivity pathway.
In application, the protection device can be used as a current interrupt to protect against excessive current passing through electrical device or apparatus. The protection device is also adaptable for use in a battery cell environment to protect against thermal run-away or overcharging. In battery cell applications, the protection device incorporates a pressure responsive mechanism for relief of excessive pressure build-up within the battery cell.